The present invention relates to a control system for an automatic transmission.
A typical prior art automatic transmission includes a torque converter acting as a fluid transmission unit for receiving the rotation generated by the engine and a speed change unit for changing the speed of the rotation transmitted from the torque converter. The speed change unit includes a planetary gear unit and changes the speed in accordance with the shift pattern which is set in advance according to the vehicle speed, the throttle opening and so on.
The conventional automatic transmission enables selection of ranges including a P (parking) range, a R (reverse) range, a N (neutral) range, a D (drive) range, a S (second) range and a L (low) range. If the range is changed from the N-range to the D-range by the shift lever, for example, the idling rotation of the engine is transmitted through the torque converter to the gear change unit, to cause a creep phenomenon wherein the vehicle inches forward without depression of the accelerator pedal.
Therefore, provision is made for preventing this creep phenomenon by releasing a forward clutch, i.e. a first clutch to be applied in forward running. In the prior art, therefore, the release of the first clutch is effected by reducing the oil pressure of the hydraulic servo of the first clutch when one of the D-range, the S-range and the L-range (hereinafter "forward running range") for moving the vehicle forward is selected but the vehicle is substantially at a stop.
However, if the first clutch is released, the vehicle may roll backward on an uphill grade against the will of the driver. Thus, simultaneously with release of the first clutch, a hill-hold control is effected, e.g. as disclosed in U.S. Pat. No. 4,648,289.
In such a hill-hold control, the shifting brake of the speed change unit is applied to block the backward rotation of the output shaft by the action of a one-way clutch to thereby prevent the vehicle from rolling backward.
In the prior art control system for an automatic transmission, however, for the hill-hold control, the time required for the piston of the hydraulic servo to move to the point of starting application of the brake, after initiation of the feed of the oil pressure to the hydraulic servo, represents a time lag before the hill-hold control becomes actually effective after the start of the application of the brake. This time lag is longer than the time period beginning with the start of the release of the first clutch and ending with lowering of the torque transmission upon start of transition of the first clutch from the applied state to the slipping state. Therefore, if the hill-hold control is started on an uphill road, simultaneously with the establishment of the release of the first clutch, the transition of the first clutch to the slipping state may start before the application of the brake is started to actually effect the hill-hold control, thereby allowing the vehicle to roll backward.
A driver will often reduce pressure on the foot brake as the creeping force acts uphill. Therefore, if the first clutch substantially fails to transmit torque before the hill-hold control becomes effective, the vehicle may possibly move backward a short distance even with maintaining a constant foot braking force. Still worse, if the application of the brake is started to effect the hill-hold control while the vehicle is moving backward, a shock is caused by the application of the brake.